Emissions of the main ground-level ozone precursor pollutants have decreased across the EEA-33 region between 1990 and 2011; nitrogen oxides (NO X ) by 44%, non-methane volatile organic compounds (NMVOC) by 57%, carbon monoxide (CO) by 61%, and methane (CH 4 ) by 29%.
This decrease has been achieved mainly as a result of the introduction of catalytic converters for vehicles, which has significantly reduced emissions of NO X and CO from the road transport sector, the main source of ozone precursor emissions.
The EU-28 as a whole reported 2011 emissions at 4% below the 2010 NECD ceiling for NO X , one of the two ozone precursors (NO X and NMVOC) for which emission limits exist under the EU's NEC Directive (NECD). Total NMVOC emissions in the EU-28 were 22% below the 2010 NECD limit in 2011, however, seven of individual Member States did not meet their ceilings for one or both of these two pollutants.
Of the three non-EU countries having emission ceilings for 2010 set under the UNECE/CLRTAP Gothenburg protocol (Liechtenstein, Norway and Switzerland), all reported NMVOC emissions in 2011 that were lower than their respective ceilings, however Liechtenstein and Norway reported 2011 NO X emissions higher than their ceiling for 2010.

Data on emissions of air pollutants (NH3, NMVOC, NOX, SO2) reported annually by Member States to the European Commission (with copies to EEA) under Directive 2001/81/EC of the European Parliament and of the Council on National Emission Ceilings for certain pollutants.

Ground-level ozone exceeded legal limits in every Member State and at many individual measurement sites during summer 2013, according to the European Environment Agency's annual report on this harmful pollutant. Although the number of exceedances is high, they have decreased over recent decades, the report notes.